1. Field of the Invention
The present invention relates to the construction of an electro-optical device. The present invention particularly relates to an active matrix type electro-optical device having a thin film transistor (TFT) made on an insulating substrate.
2. Related Background Art
In recent years, an EL display has been attracting attentions as a flat panel display that is to replace an LCD (liquid crystal display), and is actively researched. In the specification, the EL display has an EL element which is also called a light emitting device or a light emitting diode. Further, the EL (Electro Luminescence) includes triplet-based light emission or singlet-based light emission.
There are generally two types of driving system for the LCD display. One type is a passive matrix type used in an STN-LCD, etc. The other type is an active matrix type used in a TFT-LCD, etc. Similarly, there are generally two kinds of driving systems in the EL display. One type is a passive matrix type, and the other type is an active matrix type.
In the case of the passive matrix type, a wiring to serve as an electrode is arranged in each of upper and lower portions of an EL element. A voltage is sequentially applied to the wirings, and an electric current flows through the EL element so that the EL element is lighted.
In contrast to this, in the case of the active matrix type, each pixels has a TFT, and a signal can be held within each pixels.
FIGS. 15A and 15B show a constructional example of the active matrix type electro-optical device used in the EL display. FIG. 15A is a view showing the construction of the entire circuit in which a pixel portion is arranged in the center of this circuit. A gate signal line side driving circuit for controlling the operation of a gate signal line is arranged to the left of the pixel portion. A source signal line side driving circuit for controlling the operation of a source signal line is arranged above the pixel portion. In FIG. 15A, a portion surrounded by a dotted line frame shows a circuit of one pixel. FIG. 15B shows an enlarged view of this circuit. In FIG. 15B, reference numeral 1501 designates a TFT (hereinafter called a switching TFT) functioning as a switching element when a signal is written into a pixel. In FIG. 15B, the switching TFT has a double gate structure, but may also have a single gate structure, a triple gate structure or a multi-gate structure having more than three gates. One of polarities of the TFT may be selected in accordance with a constructional form of the circuit. Reference numeral 1502 designates a TFT (hereinafter called an EL driving TFT) functioning as an element (an electric current control element) for controlling an electric current supplied to an EL element 1503. In FIG. 15B, the TFT 1502 is arranged between an anode 1509 of the EL element 1503 and an electric current supply line 1507. In an alternative constructional method, it is also possible to arrange the TFT 1502 between a cathode 1510 of the EL element 1503 and a cathode electrode 1508. One of polarities of the TFT may be selected in accordance with the constructional form of the circuit. In this case, a system is common and often used in which a p-channel type TFT is used for the EL driving TFT, and the EL driving TFT is arranged between the anode 1509 of the EL element 1503 and the electric current supply line 1507, since source grounding is preferable as the operation of a transistor, and there is a restriction in manufacture of the EL element 1503. Reference numeral 1504 designates a holding capacitor for holding a signal (voltage) inputted from a source signal line 1505. One terminal of the holding capacitor 1504 in FIG. 15B is connected to the electric current supply line 1507, but there is also a case in which dedicated wiring is used. A gate terminal of the switching TFT 1501 is connected to a gate signal line 1506, and a source terminal of this TFT 1501 is connected to the source signal line 1505. A drain terminal of the EL driving TFT 1502 is connected to the anode 1509 of the EL element 1503, and a source terminal of this TFT 1502 is connected to the electric current supply line 1507.
An operation of the circuit of the active matrix type electro-optical device will next be explained with reference to FIGS. 15A and 15B. First, when the gate signal line 1506 is selected, a voltage is applied to a gate of the switching TFT 1501, and the switching TFT 1501 attains a turned-ON state. Thus, a signal (voltage) of the source signal line 1505 is accumulated in the holding capacitor 1504. The voltage of the holding capacitor 1504 becomes a voltage VGS between the gate and the source of the EL driving TFT 1502 so that an electric current according to the voltage of the holding capacitor 1504 flows through the EL driving TFT 1502 and the EL element 1503. As a result, the EL element 1503 is lighted.
Luminance of the EL element 1503, i.e., an electric current amount flowing through the EL element 1503 can be controlled by VGS. VGS is the voltage of the holding capacitor 1504, and is a signal (voltage) inputted to the source signal line 1505. Namely, the luminance of the EL element 1503 is controlled by controlling the signal (voltage) inputted to the source signal line 1505. Finally, the gate signal line 1506 is set to a not-selected state, and the gate of the switching TFT 1501 is closed, and the switching TFT 1501 is set to a turned-OFF state. At that time, electric charges accumulated in the holding capacitor 1504 are held. Accordingly, VGS is held as it is, and an electric current according to VGS continuously flows through the EL driving TFT 1502 and the EL element 1503.
The descriptions above are reported in SID99 Digest:P372: “Current Status and future of Light-Emitting Polymer Display Driven by Poly-Si TFT”, ASIA DISPLAY 98:P217: “High Resolution Light Emitting Polymer Display Driven by Low Temperature Polysilicon Thin Film Transistor with Integrated Driver”, Euro Display99 Late News:P27: “3.8 Green OLED with Low Temperature Poly-Si TFT”, etc.
In the active matrix type electro-optical device, it is required that the pixel has a large holding capacity and high aperture ratio in view of display performance of this device. Since each pixel has the high aperture ratio, utilization efficiency of light is improved and a display unit can be saved in power and made compact.
In recent years, the pixel is reduced in size and an image with higher definition is required. Since the pixel size is reduced, regions for forming the TFT and wiring come to occupy increased area in one pixel, and the aperture ratio of the pixel is reduced.
Therefore, efficient layout of circuit elements that are required in the circuit construction of the pixel is indispensable to obtain a high aperture ratio of each pixel in the prescribed pixel size.
As mentioned above, a new pixel construction that has not conventionally been found is needed to realize the active matrix type electro-optical device having a high pixel aperture ratio with a reduced mask number.